CWE 590

The application calls free() on a pointer to memory that was not allocated using associated heap allocation functions such as malloc(), calloc(), or realloc().

Extended Description

When free() is called on an invalid pointer, the program's memory management data structures may become corrupted. This corruption can cause the program to crash or, in some circumstances, an attacker may be able to cause free() to operate on controllable memory locations to modify critical program variables or execute code.

Time of Introduction

Implementation

Common Consequences

Scope

Effect

Authorization

There is the potential for arbitrary code execution with privileges of the vulnerable program via a "write, what where" primitive.

If pointers to memory which hold user information are freed, a malicious user will be able to write 4 bytes anywhere in memory.

Demonstrative Examples

Example 1

In this example, an array of record_t structs, bar, is allocated automatically on the stack as a local variable and the programmer attempts to call free() on the array. The consequences will vary based on the implementation of free(), but it will not succeed in deallocating the memory.

(Bad Code)

Example Language: C

void foo(){

record_t bar[MAX_SIZE];

/* do something interesting with bar */

...

free(bar);

}

This example shows the array allocated globally, as part of the data segment of memory and the programmer attempts to call free() on the array.

(Bad Code)

Example Language: C

record_t bar[MAX_SIZE]; //Global var

void foo(){

/* do something interesting with bar */

...

free(bar);

}

Instead, if the programmer wanted to dynamically manage the memory, malloc() or calloc() should have been used.

(Good Code)

void foo(){

record_t *bar = (record_t*)malloc(MAX_SIZE*sizeof(record_t));

/* do something interesting with bar */

...

free(bar);

}

Additionally, you can pass global variables to free() when they are pointers to dynamically allocated memory.

(Good Code)

record_t *bar; //Global var

void foo(){

bar = (record_t*)malloc(MAX_SIZE*sizeof(record_t));

/* do something interesting with bar */

...

free(bar);

}

Potential Mitigations

Phase: Implementation

Only free pointers that you have called malloc on previously. This is the recommended solution. Keep track of which pointers point at the beginning of valid chunks and free them only once.

Phase: Implementation

Before freeing a pointer, the programmer should make sure that the pointer was previously allocated on the heap and that the memory belongs to the programmer. Freeing an unallocated pointer will cause undefined behavior in the program.

Phases: Implementation; Operation

Use a library that contains built-in protection against free of invalid pointers, such as glibc.

Phase: Architecture and Design

Use a language that provides abstractions for memory allocation and deallocation.

Phase: Testing

Use a tool that dynamically detects memory management problems, such as valgrind.

In C++, if the new operator was used to allocate the memory, it may be allocated with the malloc(), calloc() or realloc() family of functions in the implementation. Someone aware of this behavior might choose to map this problem to CWE-590 or to its parent, CWE-762, depending on their perspective.